I. Field of the Invention
This invention relates generally to aircraft instrumentation for indicating the altitude of an aircraft relative to the earth's surface and more particularly to a radar altimeter which includes apparatus for calibrating the instrument so as to maintain altitude accuracy under varying operating conditions.
II. Discussion of the Prior Art
A typical prior art aircraft radar altimeter comprises a transmitter which is periodically triggered to generate a pulse of electromagnetic energy which is radiated from a transmitting antenna and a receiver, operatively coupled to a receiving antenna, for receiving an echo or return pulse from a target (ground). Included in the receiver is a closed-loop servo tracker which functions to position a range gate at the leading edge of the return pulse. Further means are provided for measuring the time interval between the transmitted pulse and the range gate, this interval being a measure of the distance to the target.
It is known that such things as temperature variations, return signal strength and range gate width can affect the accuracy of the radar altimeter. Specifically, temperature variations can impact the accuracy of the radar delay timing circuitry as well as various threshold references used in the closed-loop servo tracker. Variations in the signal strength of the return signal and the resultant signal-to-noise variance are known to affect the point on the leading edge of the return signal at which the closed loop tracker positions the range gate.
In the past as taught in the Pile U.S. Pat. No. 3,554,996, these variances have been corrected by providing a test signal at a fixed delay and a fixed amplitude to represent ground return. The altimeter then processes the simulated return and the resulting altitude is compared with the fixed delay to derive the error magnitude. The resulting error is then used to correct the altitude at all altitudes in this group of altitudes. These closed loop systems are only slightly more accurate than earlier open-loop compensated systems because the test signal is set at discrete fixed range delays, not at the actual aircraft altitude, and the test signal amplitude is also set to a pre-determined fixed level at these test altitudes, not at the actual ground return amplitude. When it is considered that it is most essential to safe flying that indicated altitudes be highly accurate at lower flying altitudes, these prior art systems are deficient.
In order to compensate for errors in the altimeter altitude time delay reference, the simulated target must be placed at the present altitude. Further, in order to compensate for altimeter gate overlap errors on the ground return, the signal strength of the simulated target must correspond to the ground return amplitude, which can vary as much as 50 dB or 100,000 to 1.
It is accordingly a principal object of the present invention to provide an automatic, closed-loop, self-calibration system for appropriately correcting indicated radar target range with particular application to radar altimeter designs requiring high accuracy and to thereby eliminate the need for manual calibration to compensate for extreme variations in temperature and return signal strength by placing the simulated target at the actual aircraft altitude delay and adjusting the amplitude of the simulated target to the actual ground return amplitude.